Ultra high capacity optical transmission systems require high spectral efficiency due to finite bandwidth of optical fibers. High spectral efficiency not only leads to larger aggregate capacity but also provides better tolerance to chromatic dispersion and polarization-mode dispersion (PMD). Spectral efficiency of modulation formats can be increased by using multilevel modulation. A preference for spectral efficient transmission systems is direct detection to allow simple receiver structures free of local oscillators and polarization control.
Optical differential quadrature phase shift keying (DQPSK) with direct detection reduces the spectral width by one half. DQPSK is a digital modulation technique commonly used with cellular systems. Motorola's CyberSurfr cable modem uses DQPSK to carry data upstream from the subscriber's computer to the Internet on a narrower frequency band than standard QPSK. Narrower bands allow more upstream channels, so the CyberSurfr has additional noise-free channels to choose from when it's installed. High spectral efficiency not only leads to larger aggregate capacity, but also provides better tolerance to chromatic dispersion and polarization mode dispersion (PMD). Direct detection on the other hand, allows simple receiver structures free of local oscillators and polarization control. Significant increase in chromatic dispersion tolerance and robustness against fiber nonlinearities in optical DQPSK transmission have been demonstrated while allowing simple direct detection.
For further spectral efficiency improvements, an 8-level amplitude and phase-shift keying (8-APSK) with direct detection combines binary amplitude-shift keying and DQPSK. This approach is a straightforward combination of two well-known modulation formats, directly yielding three binary output data sequences identical to the three input data sequences.
In single-channel 8-APSK systems, the system performance is mainly limited by different amounts of self-phase modulation (SPM) accumulated for bits with two different amplitudes. This limitation due to SPM can be mitigated through post-transmission nonlinear phase compensation for single channel systems. However, the effectiveness of post-transmission nonlinear phase compensation is limited for multi-channel systems as inter-channel cross-phase modulation (XPM) becomes major transmission impairment. Since inter-channel XPM increases with the increase of spectral efficiency (decrease of channel spacing), an 8-level modulation formats that is robust to XPM is needed. Furthermore, in order to make direct-detection more tolerant to both SPM and XPM, constant-amplitude 8-level modulation format is required.
The apparatus, system and method of the present invention is a practical implementation of constant-intensity optical differential 8-level phase-shift keying (OD8PSK) with direct detection for high-capacity spectrally-efficient fiber-optic transmission. Electrical encoding and optical encoding/modulation schemes preserve the same simplicity of received architectures based on interferometric demodulation and direct detection, directly yielding three binary output data sequences identical to the three input data sequences. The present invention includes three different optical encoding/modulation schemes and the corresponding electrical encoders.